In fluid systems there is a need for accurate and reliable valves to control flow and balance system loads. This need exists in all types of systems for low pressure and flow, domestic, general, commercial, industrial and high pressure systems more particularly in industrial chemical and nuclear applications.
In the present state of the art, electro-mechanical, hydraulic or pneumatic actuators are cumbersome, frequently unreliable over the complete open-to closed range, and fail to provide a repeatably accurate positioning of the plug and rapid response to calls for flow variations.
Presently available flow control valves have packed stems which require a build-up of force (normally provided by solenoid, or pneumatic actuators) in order to overcome packing friction on the valve stem which leads to sluggish response and jerking motion of the valve plug. In the case of solenoid or pneumatic operators, the plug is snapped closed by fluid flow when the actuator brings the plug toward the closed position.
In the case of large control valves in high pressure applications, the large material mass of a valve bonnet and actuator offset in many cases several feet from the center line of the pipe is of grave concern to the design engineer who must provide for suitable hangers and snubbers to stabilize the non-symmetric mass in the event of severe earthquake activity. This concern is doubly important in the design of chemical and nuclear stations.
Bellows sealed control valves are not widely used because of the necessity to provide bonnet containment of a very long bellows to provide reasonable life expectancy in view of the frequent motion of a control valve stem.
A control valve design which provides for rapid response and a plug actuator capable of a smooth, even motion is not presently available.